The invention generally relates to fuel controls and more particularly to fuel controls for turboshaft engines. This application has matured into U.S. Pat. No. 3,721,453. In the past, fuel controls have embodied various devices to produce fuel flow acceleration schedules for their associated engines. Notable among these prior art devices is the well-known three-dimensional cam. While prior controls, incorporating three-dimensional cams, yield satisfactory performance, they are relatively expensive to manufacture. Additionally, prior art fuel systems have not always been endowed with the requisite degree of flexibility necessary to meet the requirements of small gas turbine engines. Also, prior art fuel controls for turboshaft engines incorporating a gas producer control and a power turbine governor are characterized by the fact that the functioning of one unit is dependent upon the functioning of the other unit. This type of design not only limits the flexibility of the fuel control, but also detracts from operating characteristics thereof.
Prior art control system with speed governors usually employ a spring to transmit a force which corresponds to a requested speed. Typically, a cam contoured throttle level shaft contacts an end of the spring while the other end is applied to a speed sensitive element which may be a set of flyweights or a pressure diaphragm element of a hydraulic speed sensor. This arrangement has the disadvantage of producing a governor having a high spring rate which has an adverse effect on the governor's speed holding accuracy. Furthermore, a high spring rate causes a substantial reaction force on the throttle level, thereby promoting wear, increasing throttle torque, and tending to return the throttle to a low speed setting in the event of a throttle linkage failure.
Also, prior art governors, which do not include an integrating feature, have not provided for isochronous or constant speed operation during steady state operation at various ambient temperatures and altitudes. Neither a conventional corrected speed nor actual speed droop governors are capable of maintaining engine speed in such a manner. It is highly desirable to have a fuel control system which is adapted to maintain a constant gas producer speed, irrespective of changes in ambient temperature and altitude since it allows a pilot to set an unvarying gas producer speed.